Nitride film

ABSTRACT

A III nitride film is directly grown on a crystalline substrate along the C-axis, and includes at least Al element. Then, the III nitride film has hexagonal crystal system, and the lattice constant “c” of the main axis and the lattice constant “a” of the crystal face perpendicular to the main surface satisfies the relation of “C&gt;2.363a-3.232”.

BACKGROUND OF THE INVENTION

[0001] (1) Field of the Invention

[0002] This invention relates to a III nitride film, particularly to anAl-including III nitride film which is suitable for a light-emittingdiode or a high speed IC chip.

[0003] (2) Related Art Statement

[0004] Al-including III nitride films are used as semiconductor filmsfor light-emitting diodes, and recently get attention as semiconductorfilms for high speed IC chips of cellular phones.

[0005] Conventionally, such an Al-including film is fabricated by aMOCVD method in which trimethyl aluminum (TMA) and triethyl aluminum(TEA) are employed as an Al supply source, and ammonia (NH₃) is employedas a nitrogen supply source.

[0006] In this case, a substrate on which the III nitride film is formedis set on a susceptor installed in a reactor and is heated to around1000° C. by a heater provided in or out of the reactor. Then, the Alsupply source, the nitrogen supply source and another supply sourceincluding another additive element, with given carrier gases, areintroduced into the reactor and supplied onto the substrate.

[0007] Just then, the supply sources thermochemically react on thesubstrate, and the thus decomposed constituent elements chemically reactto deposit a desired Al-including III nitride film on the substrate.

[0008] However, when the Al-including III nitride film is formeddirectly on a single crystal substrate made of C-faced sapphire singlecrystal, C-faced SiC single crystal or the like or an epitaxialsubstrate constructed of the same single crystal and an underfilm madeof III nitride, some cracks may be created in the film, irrespective ofthe composition, depending on the film-forming condition and thefilm-forming apparatus condition such as the reactor configuration andsize.

[0009] Therefore, the yield ratio of elements constructed of such singlecrystal substrates or epitaxial substrates and the Al-including IIInitride films formed on the substrates is degraded.

SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to provide anAl-including III nitride film without cracks when it is formed directlyon a single crystal substrate or an epitaxial substrate.

[0011] In order to achieve the above object, this invention relates to aIII nitride film including at least Al element and having hexagonalcrystal system which is directly grown on a crystalline substrate alongthe C-axis, the lattice constant “c” of the main axis and the latticeconstant “a” of the crystal face perpendicular to the main surfacesatisfying the relation of “c>2.363a-3.232”. Here, “along the C-axis”includes subtle tilt of the C-axis originated form deviation of growthconditions, subtle off-angle value of crystalline substrate and soforth.

[0012] In a preferred embodiment of the present invention, the latticeconstant “c” and the lattice constant “a” satisfy the relation of“c>2.636a-3.221”. In this case, even though the III nitride film isthermally shocked, no cracks are created in the film.

[0013] The inventors had intensely studied to obtain an Al-including IIInitride film without cracks when it is formed directly on a crystallinesubstrate. Then, since the cracks were created, irrespective of thecomposition, depending on the film-forming condition and thefilm-forming apparatus condition, they also had intensely investigatethe physical properties to influence the crack creation.

[0014] As a result, they found out that the crack creation of the IIInitride film directly formed on the crystalline substrate relates to thesizes of lattice constants of the crystal lattice of the III nitridefilm. That is, in the hexagonal crystal lattice of the III nitride film,if the lattice constant “c” of the main axis and the lattice constant“a” of the crystal face perpendicular to the main axis satisfy the aboverelation, the crack creation can be inhibited even though the IIInitride film is formed directly on the crystalline substrate. Thisinvention results from the above enormous research and development.

[0015]FIG. 1 is a graph showing the correlation between the latticeconstants and the crack creation in a III nitride film having acomposition of AlxGayInzN (x+y+z=1, x>0). As is apparent from FIG. 1, ifthe relation of “c>2.636a-3.232” is satisfied, cracks are not created inthe film. On the other hand, if the relation is not satisfied, somecracks are created in the film.

[0016] Herein, in FIG. 1, ∘ plots designate “not crack creation”, and ×plots designate “crack creation”.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] For better understanding of the present invention, reference ismade to the attached drawings, wherein

[0018]FIG. 1 is a graph showing the correlation between the latticeconstants and the crack creation in a III nitride film having acomposition of AlxGayInzN (x+y+z=1, x>0).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] This invention will be described in detail, hereinafter.

[0020] It is required in the present invention that in the hexagonalcrystal lattice of a c-axis grown Al-including III nitride film, thelattice constant “c” of the main axis(c-axis) and the lattice constant“a” of the crystal face perpendicular to the main axis satisfy therelation of “c>2.636a-3.232”, preferably “c>2.636a-3.221”

[0021] The above requirement can be realized by controllingappropriately the temperature, the pressure and the supply gas sourceratio, and controlling appropriately the evacuating rate and the flowdirection of the supply gas on a crystalline substrate on which the IIInitride film to be formed. These conditions are appropriately selectedand optimized, depending on the material, the composition and thethickness of the III nitride film to be formed.

[0022] Moreover, the above requirement can be satisfied by forming anunderfilm between the crystalline substrate and the III nitride film andoptimizing the composition of the underfilm. For example, a high qualitysingle crystalline III nitride film having a large Al content is formedas the underfilm on a C-faced sapphire substrate, and the III nitridefilm is formed on the single crystalline nitride film.

[0023] The III nitride film is required to include at least Al element.In the case that the III nitride film is used as a semiconductor filmfor a light-emitting element, it is desired that the film has thecomposition of AlxGayInzN (x+y+z=1, x>0). Moreover, the III nitride filmmay contain an additive element such as B, Si, Mg.

[0024] Moreover, the III nitride film has preferably larger Al content.Concretely, the film preferably has the composition of AlxGayInzN(x+y+z=1, x>0.5), particularly AlxGayInzN (x+y+z=1, x>0.9).

[0025] Such III nitride films can be employed for light-emitting diodesand high speed IC chips. In this case, the III nitride films are formeddirectly on their respective crystalline substrates, so some cracks arecreated in the films, depending on their film-forming conditions andfilm-forming apparatus conditions.

[0026] Therefore, if the nitride films satisfy the above latticeconstant relation according to the present invention, cracks are almostnever created in the films. As a result, the yield ratio of suchlight-emitting diodes and high speed IC chips can be developed.

[0027] It is desired that the thickness of the III nitride film is setto 1 μm or over, particularly 2 μm or over. Conventionally, if thethickness of a III nitride film as formed directly on a crystallinesubstrate is larger than 1 μm, a large amount of crack may be created inthe film. However, if the thicker film satisfies the above latticeconstant relation, the crack creation can be inhibited effectively.Therefore, according to the present invention, a thicker III nitridefilm can be provided.

[0028] The crystalline substrate may be made of any single crystal.Concretely, oxide single crystal such as sapphire single crystal, ZnOsingle crystal, LiAlO₂ single crystal, LiGaO₂ single crystal, MgAl₂O₄single crystal, or MgO single crystal, IV single crystal or IV-IV singlecrystal such as Si single crystal or SiC single crystal, III-V singlecrystal such as GaAs single crystal, AlN single crystal, GaN singlecrystal or AlGaN single crystal, and boride single crystal such as Zr₂B₂are exemplified.

[0029] Moreover, an epitaxial substrate constructed of a single crystalsubstrate made of the above-mentioned single crystal and an underfilmformed on the substrate may be employed. For facilitating the epitaxialgrowth of the Al-including III nitride film of the present invention,the underfilm also is preferably made of an Al-including III nitridefilm.

[0030] Then, in the case that the Al-including III nitride film of thepresent invention has the composition of AlxGayInzN (x+y+z=1, x>0), forthe facilitation of the epitaxial growth, it is desired that theunderfilm also has the composition of AlaGabIncN (a+b+c=1, a>x).

EXAMPLES Example

[0031] This invention will be concretely described, hereinafter.

[0032] A C-faced sapphire single crystal substrate was employed, andthen, set and attracted on a susceptor installed in a quartz reactor.Then, trimethyl aluminum (TMA), trimethyl gallium (TMG) and trimethylindium (TMI) were employed as an Al supply source, a Ga supply sourceand an In supply source, respectively. Moreover, ammonia gas wasemployed as a nitrogen supply source. Since those supply sources exceptammonia gas are liquid or solid at room temperature, they are vaporizedthrough bubbling operation.

[0033] Then, the ammonia gas which corresponds to V element supplysource and the TMA, the TMG and the TMI which correspond to III elementsupply source were introduced into the reactor at a (V/III) flow ratioof 1000 or below, and supplied onto the substrate. Then, the substratewas heated to 1150° C., and thus, an AlGaInN single crystalline film wasfabricated as an underfilm in a thickness of 1 μm. When thecrystallinity of the AlGaInN underfilm was investigated by X-rayanalysis, it was turned out that the full width at half maximum was 90seconds or below, so that the underfilm has a high crystallinity.

[0034] Thereafter, the substrate having the AlGaInN underfilm was heatedto 1200° C., and subsequently, cooled down to a given temperature lowerthan the underfilm-forming temperature of 1150° C. Then, the same supplysources were supplied at a higher (V/III) flow ratio than theunderfilm-forming flow ratio of 1000 or below to fabricate pluralAlGaInN films. These results include underfilm data.

[0035] The lattice constants of the AlGaInN films were measured by X-raydiffraction, and their respective lattice constants “c” and “a” wereplotted as 0 plots. These date include the underfilm data. And, anycracks were not created in all of the AlGaInN films.

Comparative Example

[0036] The (V/III) flow ratio and the substrate temperature beingcontrolled appropriately, an AlGaInN underfilm having a smaller Alcontent than an AlGaInN film to be formed was formed on a C-facedsapphire substrate. Then, the AlGaInN film was formed on the underfilm.A film-forming process like this repeated twice, two kinds of AlGaInNfilm were fabricated.

[0037] The lattice constants “c” and “a” of the AlGaInN films weremeasured by the same manner as in Example, and plotted as X plots. Manycracks were created in the AlGaInN films.

[0038] As is apparent from FIG. 1 and the above-mentioned crackobservation, in the AlGaInN films satisfying the relation of“c>2.636a-3.232” for the lattice constant “c” and “a”, no cracks werecreated. On the other hand, in the AlGaInN films not satisfying theabove relation for the lattice constant “c” and “a”, many cracks arecreated.

[0039] Moreover, the obtained films were exposed in the high temperatureof 1000° C. in order to do thermal shock test. After the test, theAlGaInN films satisfying the relation of “c>2.636a-3.221” for thelattice constant, “c” and “a”, no cracks were created. However, in somefilms satisfying the relation of “2.636a-3.221≧c>2.636a-3.232”, crackswere generated.

[0040] Although the present invention was described in detail withreference to the above examples, this invention is not limited to theabove disclosure and every kind of variation and modification may bemade without departing from the scope of the present invention.

[0041] As explained above, according to the present invention, if a IIInitride film including at least Al element is formed directly on acrystalline substrate such as single crystal substrate made of C-facedsapphire single crystal or C-faced SiC single crystal, or an epitaxialsubstrate, no cracks are created in the film.

What is claimed is:
 1. A III nitride film including at least Al elementand having hexagonal crystal system which is directly grown on acrystalline substrate along the C-axis, the lattice constant “c” of themain axis and the lattice constant “a” of the crystal face perpendicularto the main surface satisfying the relation of “c>2.363a-3.232”.
 2. AIII nitride film as defined in claim 1, wherein the lattice constants“c” and “a” satisfies the relation of “c>2.636a-3.221”.
 3. A III nitridefilm as defined in claim 1, wherein the III nitride film has thecomposition of AlxGayInzN film (x+y+z=1, x>0.5).
 4. A III nitride filmas defined in claim 3, wherein the III nitride film has the compositionof AlxGayInzN film (x+y+z=1, x>0.9).
 5. A III nitride film as defined inclaim 1, wherein the thickness of the III nitride film is set to 1 μm orover.
 6. A III nitride film as defined in claim 1, wherein thecrystalline substrate is made of a single crystal substrate.
 7. A IIInitride film as defined in claim 6, wherein the single crystal substrateis a C-faced sapphire substrate or a C-faced SiC substrate.
 8. A IIInitride film as defined in claim 1, wherein the crystalline substrate ismade of an epitaxial substrate which is constructed of a given basematerial and a III nitride underfilm including at least Al element.
 9. AIII nitride film as defined in claim 8, wherein the III nitrideunderfilm has the composition of AlaGabIncN (a+b+c=1, a>x).
 10. A IIInitride film as defined in claim 9, wherein the crystallinity of the IIInitride underfilm is set to the full width at half maximum of 90 secondsor below.